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### Topic: Lowering voltage by a specific amount? (Read 991 times)previous topic - next topic

#### patrickc01

##### May 30, 2012, 01:10 am
I bought a sparkfun laser card (http://www.sparkfun.com/datasheets/Components/Laser-Card.pdf)
It states that the voltage should be 3.1 +/- 10%.  I was trying to figure out how I can lower the voltage from 3.5 (or 5V) to 3.1.  Is there a way to adjust voltage like this?  Or would it be ok to put a diode in the circuit (from what I understand this would lower the voltage by ~.7).
Thanks for any help!

#### majenko

#1
##### May 30, 2012, 01:36 am
It all depends on how much current draw you're looking at, and how precise you want to get the voltage.

You can use an adjustable "Low Drop-Out" voltage regulator to get a precise voltage from the 5v connection.  This would be the most precise way.

Yes, a diode will give a roughly 0.7v drop per diode.  You can't get 3.1v using silicon diodes alone.  A combination of silicon and schottky diodes could be employed, but that would probably cost more than an LDO regulator.

If the current draw is a constant value (say for an LED), then a simple resistor can be used to drop the voltage.  Using Ohm's law you can calculate the value of the resistor to use.

#### patrickc01

#2
##### May 30, 2012, 04:18 am
Thanks for the answer.  I was under the impression that resistors lower current, not voltage, guess I was wrong..  Havent heard about schottky diodes, putting it on my list of things to learn

#### Jack Christensen

#3
##### May 30, 2012, 04:24 am

Thanks for the answer.  I was under the impression that resistors lower current, not voltage, guess I was wrong..  Havent heard about schottky diodes, putting it on my list of things to learn

Resistance is the constant of proportionality between voltage and current. See http://en.wikipedia.org/wiki/Ohms_law

#### Techylah

#4
##### May 30, 2012, 05:22 am
Use a germanium diode, like a 1N34A, instead of silicon.   It has a drop of .15 to .3v depending on current.  Perhaps use two.

#### Magician

#5
##### May 30, 2012, 06:29 am
Resistor should be o'k for laser, knowing current from the spec. data, do a math : R = ( 5 - 3.1 ) / 35 mA = 54.28 Ohm

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